There is now strong evidence that mammalian milk, particularly that formed during the early postpartum period, is more than just a source of nutrition an immunoglobulins for the young. In fact, it is also a concentrated "cocktail' of hormones and growth factors that has the potential to regulate the development and growth of the neonate. Many of these hormonal agents are found at high concentrations in milk, an are absorbed and efficiently transferred to the neonatal circulation in biologically active forms. Surprisingly, there is a dearth of direct evidence to support a requisite physiologic role for these absorbed hormones in neonatal development. Our group has recently obtained strong evidence that a low molecular weight, milk peptide regulates the differentiation of PRL- secreting cells in neonatal rats. Our goal now becomes one of identifying the milk-borne factor that initiates this process, elucidating the mechanism by which it acts, and establishing its physiologic role in normal development. We should like to stress, however, that our proposed studies have even broader implications in that our approach should be viewed as a prototypic strategy for establishing whether any maternal signal delivered via milk plays a requisite role in infant growth and development. Information derived from these studies will be essential for fully evaluating the importance of breast feeding and for the rational design of supplements to human infant formulas.